The present invention relates generally to the field of process control instruments employing vibratable elements with resonant frequencies that change in accordance with externally imposed influences, and more particularly to the drive mechanism for such instruments.
It is well known in the process control industry that accurate and effective sensors can be made for detecting changes in process variables such as the rate of fluid flow, pressure, temperature, etc., by making use of the oscillatory characteristics of a vibrating element such as a quartz crystal, a reed, a wire or a flat ribbon. In the case of the vibrating wire or ribbon, which can be described by the more general term "strings" (i.e., stretched elements fixed at both ends), vibration typically is induced by the electromagnetic interaction between a magnetic coil assembly and an electric current passing through the string. Examples of such electromagnetic schemes are shown in U.S. Pat. No. 4,165,651, and copending applications Ser. Nos. 155,325 and 161,123, which have the same assignee as the present application. Mechanical drive means also have been envisioned, such as the arrangements disclosed in copending patent application Ser. No. 365,678, filed Apr. 5, 1982. However, no resonant element process control instrument has been found in which the vibrating string is stimulated by means of an air discharge.
The classical wind-driven string instrument known as the Aeolian harp utilized the passage of air over an arrangement of strings to produce a musical tone. However, this interaction between the wind and the strings was more or less haphazard, and this principle is not known to have been applied in any systematic fashion to a commercially available industrial process control instrument. U.S. Pat. No. 4,257,010 does allude to the delivery of energy to a vibratable band by pneumatic means, but without any hint of a structure to accomplish that result.
One of the drawbacks of the magnetic coil assembly or other electrically-based drive systems is that electrical potentials generally are present within the vibrating element instrument itself. Often times the instrument is located in an atmosphere containing combustible or explosive gases, which can be ignited by electrically-generated sparks. Safety requirements impose stringent limits on voltage and current levels allowed. Such limits may hamper the operating efficiency of the instrument. Also, if a user of pneumatic process control instruments wishes to employ a vibrating element device, he may need to introduce electrical circuitry into a region where previously it was not required. Naturally this entails additional expense and inconvenience.
Therefore in light of the above considerations, it is an object of the present invention to provide a drive system for inducing oscillation of a vibratable string which eliminates the presence of electrical potentials at the site of the vibratable string. In addition, it should be easily adaptable to presently existing pneumatic control systems.
It is a further object of the present invention to provide such a drive system which is energy efficient, as well as being reliable and economical to install and maintain.